This invention relates to the field of vibration isolation and shock absorption. More particularly, this invention relates to the field of vibration isolation and shock absorption in borehole telemetry, especially mud pulse telemetry wherein data relating to borehole parameters is gathered by sensing instruments located downhole in the drillstring and is transmitted to the surface via pressure pulses created in the drilling mud. The mud pulse telemetry apparatus and associated sensor apparatus are high precision equipment which are installed in a special section of the drillstring close to the drill bit or mud motor. This precision equipment must be protected from the shock and vibration caused by drilling operations.
The basic concept of mud pulse telemetry for transmitting borehole data from the bottom of a well to the surface has been known for some time. U.S. Pat. Nos. 4,021,774, 4,013,945 and 3,982,431, all of which are owned by the assignee of the present invention and which are incorporated herein by reference in their entirety, show various aspects of a mud pulse telemetry system. Mounting and shock absorber assemblies intended for use in mud pulse telemetry systems are shown in U.S. Pat. Nos. 3,714,831, 3,782,464 and 4,265,305, the latter of which is assigned to the assignee hereof and all of which are incorporated herein by reference in their entirety. The mounting and shock absorber assembly of previously mentioned U.S. Pat. No. 4,265,305 has been in commercial use in the mud pulse telemetry system operated by the assignee hereof. While the mounting and shock absorber assembly of that patent and the others referred to above are adequate for their intended purposes, they have certain drawbacks. For example, the systems of U.S. Pat. No. 3,714,831 and 3,782,464 require that the drill collar in which they are mounted be in two pieces in order to have access to the shock absorber elements for assembly. That requirement for a two piece drill collar poses several disadvantages, discussed in more detail in U.S. Pat. No. 4,265,305. While the system disclosed in U.S. Pat. No. 4,265,305 eliminates the problem of the requirement for a two piece drill collar, that system (and those of U.S. Pat. Nos. 3,714,831 and 3,782,469) requires two different kinds of elements, and it experiences a discontinuity in its load absorption characteristics. The system of U.S. Pat. No. 4,265,305 is made up of essentially two elements: a series of elastomeric rings which function as springs or isolators, and a bumper structure. The elastomeric rings have a relatively low constant spring rate, while the bumper structure has a relatively high spring rate. The isolators or elastomeric springs are characterized by relatively high displacement for relatively low forces; and the bumper structure is characterized by having relatively low displacement for relatively high forces. The shock absorber structure of U.S. Pat. No. 4,265,305 usually operates over a range in which both the elastomeric springs or isolators and the bumpers are operative. As a result, and because of the different spring rate characteristics, a potentially serious discontinuity in load bearing characteristics occurs at the transition point between operation of the elastomeric springs or isolators and the bumper structures. This discontinuity is undesirable.